Page 179 - Refining Biomass Residues for Sustainable Energy and Bioproducts

152 Refining Biomass Residues for Sustainable Energy and Bioproducts

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The toxicity of ammonia is pH dependent. The proportion of NH 3 to NH spe-
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cies in the cultivation medium increases with the pH and the relationship is
described by the Henderson Hasselbalch equation (Harris, 2017):

½ NH 3 Š
pH 5 pK a 1 log (7.2)
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[NH ]
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There are several mechanisms happening during the growth of microalgae that
affect the pH of the culture medium. One of them is the dissolution of carbon diox-
ide (CO 2 ) in the culture media, which is converted to carbonic acid and then to
bicarbonate:

1 2
CO 2 1 H 2 O2 H 2 CO 3 2H 1 HCO 3 (7.3)
Carbon dioxide Carbonic acid Bicarbonate
Algae incorporate inorganic carbon for photosynthesis mainly in the way of
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bicarbonate. The presence of bicarbonate results in the production of H ions that
reduces the pH of the culture medium. On the other hand, the photosynthesis pro-
cess elevates the pH of the cultivation medium (Lin et al., 2007). When the cell
concentration in a batch culture is sufficiently high, the rate of inorganic carbon
uptake by photosynthesis can exceed the rate of CO 2 supply from the atmosphere,
the partial pressure of CO 2 of the medium then decreases and the pH goes up.
According to Ge and Champagne (2016), a pH range of 8.53 9.21 suppresses
phototrophic growth, as at these pH levels, the availability of CO 2 decreases and
limits photosynthesis and growth of algae.
Algae cultures can also fulfill their inorganic carbon requirements from sodium
bicarbonate (NaHCO 3 ), with no reductions in the pH. The growth of Chlorella sp.,
isolated from a landfill site, was enhanced by 70% by the addition of 100 mM
NaHCO 3 to the culture medium when compared to the control in Bold Basal
Medium (BBM) (Mishra et al., 2018). However, concentrations of NaHCO 3 higher
than 200 mM led to a sharp increase in the pH which inhibited the growth of
Chlorella sp. (Mondal et al., 2017).
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An increase in the pH can also occur when NO is used as the only N source
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(Grobbelaar, 2004). In contrast, when ammonia is used as the sole source of N, the
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pH could drop significantly due to the release of H ions. Moreover, at alkaline pH
values, NH 3 could be lost by volatilization (AlMomani and Ormeci, 2016; Lin
et al., 2007).
NH 3 toxicity and assimilation can be influenced by the presence of organic car-
bon in the cultivation medium as reported by Lu et al. (2018). This phenomenon
was explained by the combination of NH 3 -N with alpha-ketoglutarate (α-KG), a
carbon intermediate in Krebs cycle, to form glutamate, a precursor of amino acids,
through the glutamine synthetase glutamine oxoglutarate aminotransferase path-
way. They compared the NH 3 assimilation using two sources of organic carbon (cit-
rate and glucose) and one source of inorganic carbon (NaHCO 3 ). They reported that
the use of organic carbon was advantageous to the cells compared to the use of